ABSTRACT
The most common lights to use for this purpose are cool-white fluorescent and high-intensity discharge (HID) lights as mentioned in Chapter 16� The measurement of light intensity in the photosynthetically active radiation was explained in Chapter 5� The amount of light units depends upon the total area of the greenhouse� For example, a 10½ ft × 12 ft greenhouse would need three 8-ft high-output coolwhite fluorescent lights, one placed above each bed of plants or six 4-ft T5 fluorescent lights� A 10½ ft wide greenhouse would fit three hydroponic beds� This is a total area of 126 ft2� Two 600-watt HID lights would provide ample light for this size of greenhouse� They would give about 550 ft candles of light intensity� Mount them a few feet under the roof of the greenhouse above the aisles between the plant beds to get maximum light distribution� They should have horizontal reflectors� Several compound light emitting diode (LED) lights could be used to save electrical consumption, but the initial cost is higher than other light sources� LED lights are available as 4-ft fixtures with either four or eight lamps� The lights can be mixed with red and blue tubes� You would need at least four 4-ft fixtures at a cost of approximately $200 each� Compact fluorescent lights are also available with red and blue mixed lamps in a reflective fixture� The lamps are available in 125-watt, 200-watt, and 300-watt sizes� The lamps and fixtures cost about $100-200 each depending upon their wattage� For a greenhouse of 126 ft2, at least four 300-watt or six 200-watt units would be needed�
To calculate the amount of lights for a specific area, use a base of 50-70 watts per square foot of growing area� For a 126 ft2 greenhouse, the actual growing beds would be two beds 2 ft × 12 ft and one bed 9 ft × 2 ft� The total growing area is 2 (2 × 12) + 1 (2 × 9) = 48 + 18 = 66 ft2� You would need 66 × 50 watts = 3300 watts� This calculation, however, is for a dark room without natural light; hence, a little less than half of the wattage (1200-1500 watts) would be sufficient for a greenhouse�
Temperature fluctuations in a greenhouse are dependent upon the outside weather conditions, unlike indoor gardening in a house with insulation� The covering of the greenhouse determines how much insulation capacity it has� However, keep in mind that light is extremely important so choose a material that has good light transmission� Common covering materials include glass, polyethylene, and polycarbonate� A double layer of polyethylene will provide more insulation than glass� Polycarbonate is available in various laminates� Double and triple layers are available to reduce heat loss� It also comes in corrugated, but that has less insulation capacity than the double or triple layers� Several disadvantages to polyethylene covering are that even with a special greenhouse film with polymers to reduce ultra violet breakdown, it will last only 3 or 4 years before replacement� Double polyethylene has an advantage of saving energy loss due to a dead air space between the two layers of film, but the disadvantage is the loss of light due to the two layers� This may be as much as 20% or more with aging of the film� Polyethylene covering also does not have the aesthetic appearance of glass or polycarbonate�
Temperature control includes heating and cooling� Due to the “greenhouse effect” as light enters a greenhouse, its wavelength is changed and the heat cannot escape, so there is a heat build-up within the greenhouse under sunlight conditions� When it is cloudy this does not occur� Even during winter months, a very bright sunny day will cause temperatures in the greenhouse to rise above optimum and then some ventilation is needed� During summer months, cooling is a key factor to maintain optimum temperatures during the day when it is sunny� This is achieved by the use of exhaust fans and, if necessary, cooling pads� In backyard hobby greenhouse due to their small area, normally exhaust fans are adequate without cooling pads, unless you are located in a very hot area with desert conditions�
Heating is needed during cool seasons during the days and nights when sunlight is not present� The best form of heat is by use of a boiler and hot water heating pipes at floor level� However, this is not economically feasible with small greenhouses and therefore space heaters are used� The best type of space heater is a unit heater mounted near the ceiling of the greenhouse, but these types of heaters are much more expensive than free-standing space heaters� The most common are electrical and gas-or propane-fired models� If you wish to use an electrical space heater, purchase a 220-volt one that is more energy efficient than a 110-volt one� There are also propane heaters that are located remote from the propane cylinder or others that are mounted on top of the cylinder� More details on heaters, calculations of sizes needed to meet temperature differentials, exhaust fans, covering materials, and so on are given in Chapter 21 on “Components for Backyard Greenhouses�”
As discussed in Chapter 16, carbon dioxide (CO2) enrichment increases production for all greenhouse crops by up to about 20%� Small CO2 generators were discussed in that chapter� I recommend using the same generators as you would for indoor culture for a backyard, hobby greenhouse� However, whenever the greenhouse is ventilating,
especially in high-light, late-spring to early-fall periods, a CO2 generator would not be needed and in fact would waste fuel during that time� Under cloudy weather the CO2 enrichment is very beneficial, so at that time have the generator operating, but only during the day period, not at night� If you use a propane-fired heater, it will naturally generate CO2, so the need for an additional CO2 generator is unnecessary�
Optimum relative humidity is about 75% for most plants� High humidity in the greenhouse during cloudy, dull, wet, weather is the main concern� If plants appear moist on their leaves, the best method to reduce the humidity is to heat and exhaust some of the heated air to exchange the air with drier air� Even if it is raining outside and the relative humidity is 100%, normally during such clement weather the ambient temperatures will be low, so heating of the greenhouse air is taking place� As the heater raises the incoming colder air, the humidity will fall� Because of this physical relationship between rising temperatures and falling relative humidity, as the greenhouse air is exhausted and replaced by cold outside air that is heated as it enters the greenhouse, the relative humidity is reduced with the incoming air�
During summer, with hot, dry weather, the greenhouse air may become very low in relative humidity� In this case, you can install some misters above the crop rows� They would be at about 3 ft centers, depending upon the specifications of the mist nozzles, along a ¾″ diameter PVC pipe� They are attached to the pipe by special “saddles” glued to the pipe� The header pipes are connected to a pump capable of 40-60 psi pressure to attain a fine mist� The system is operated by a time-clock or other controller� Misting cycles and periods depend upon the level of relative humidity� The lower or drier the conditions, the more often the mist is activated� Normally, this would be about every 15-20 min for a period of 10-15 sec� If the relative humidity is very low, plants may lose excess water causing the stomates to close, restricting the entrance of CO2 and therefore slowing photosynthesis and plant growth� Plants under such conditions can wilt and leaf margins may burn�
Drip irrigation is with a pump in the nutrient tank that is activated with a time-clock or controller� The cycles (time between irrigations and the length of time during an irrigation cycle) depend upon the weather conditions (temperature, sunlight, relative humidity), season (day length), crop stage of growth, and the nature of the crop� Lowprofile plants are normally grown in a nutrient film technique (NFT) or raft culture system so irrigation is constant� Vine crops growing in a substrate with drip irrigation require closer monitoring of irrigation cycles and adjustments according to the earlier-mentioned factors� Large-leaf plants such as European cucumbers and eggplants due to the expansive leaf area surface require more irrigation than do tomatoes or peppers having smaller total leaf area� All of these parameters of caring for your plants will be a learning experience, especially for the first crops undertaken�
However, that is the fascinating challenge of growing that will increase yields with your experience of tending to them�
Time-clocks and controllers suited for drip irrigation are discussed in Chapter 21� Whenever an irrigation cycle occurs for hydroponic systems, irrigation is always with a nutrient solution, not raw water� If plants wilt during mid-day, high-light conditions, the probable cause is lack of water� Immediately increase the irrigation cycle frequency� The duration of an irrigation cycle is also important as mentioned earlier in Chapter 6 in achieving sufficient leachate to prevent any salt build-up in the substrate�
